Exploiting science to increase oil recovery 5 - What limits sweep

Summary

The script discusses the concept of reservoir sweep in oil recovery, a critical factor affecting efficiency. It explains how water, due to its heavier nature, tends to follow the path of least resistance and permeable layers, leading to incomplete oil extraction. The script also touches on the challenges of vertical and aerial sweep, including the tendency of water to slump and the influence of high permeability channels. It concludes by hinting at the use of gas in secondary oil recovery and its limitations, piquing interest in the subject.

Takeaways

• 💧 Water injection into a reservoir is limited by the sweep efficiency, which is a measure of how effectively the oil is displaced by the injected water.
• 🗻 Permeability variations in the reservoir rock cause water to preferentially flow through more permeable layers, leading to an uneven sweep.
• 🔻 Water, being denser than oil, tends to slump down under the oil, which can lead to early breakthrough at the production well.
• 🔄 The path of least resistance and the tendency of water to follow established paths can limit the horizontal aerial sweep of the reservoir.
• 🌐 High permeability channels, such as old riverbeds, can divert water away from sweeping the rest of the reservoir, further reducing sweep efficiency.
• 📊 The total sweep of a reservoir is calculated by multiplying the vertical sweep by the aerial sweep, giving a percentage of the oil volume contacted by the injected fluid.
• 🌪 Gas injection for secondary oil recovery is even more affected by the factors impacting sweep efficiency than water injection.
• 🔄 Gas, like water, tends to follow the path of least resistance, which can lead to channeling and reduced sweep efficiency.
• 📉 The efficiency of secondary oil recovery methods, including water and gas injection, is influenced by the reservoir's geological characteristics and fluid behavior.
• 🛑 Understanding and optimizing the sweep behavior is crucial for improving oil recovery rates and extending the life of oil fields.
• 🔬 Advanced reservoir modeling and monitoring techniques can help predict and mitigate the limitations of sweep efficiency in oil recovery operations.

Q & A

What is the purpose of injecting water into a reservoir in the context of oil recovery?

The purpose of injecting water into a reservoir is to push the oil towards the production well, enhancing oil recovery by utilizing the water as a driving force.

What does 'sweep' refer to in the context of oil recovery calculations?

In oil recovery calculations, 'sweep' refers to the extent to which the injected water or other fluids are able to access and displace oil within the reservoir.

Why is the permeability of rock in a reservoir not uniform, and how does it affect the sweep?

The permeability of rock in a reservoir is not uniform due to variations in rock properties. This affects the sweep as water tends to flow through more permeable layers, following the path of least resistance and potentially bypassing less permeable areas with oil.

How does the density of water compared to oil influence its movement within the reservoir?

Since water is heavier than oil, it tends to slump down under the oil layer and can break through to the production well at the base, affecting the vertical sweep of the reservoir.

What is meant by the 'aerial sweep' of a reservoir, and how does it differ from vertical sweep?

Aerial sweep refers to the horizontal reach of the injected water in the reservoir. Unlike the vertical sweep, which is concerned with the height of the reservoir that is contacted, aerial sweep focuses on how far the water spreads out laterally.

How do higher permeability channels in the rock, such as old riverbeds, impact the reservoir sweep?

Higher permeability channels, like old riverbeds, can cause water to preferentially flow through these paths, potentially bypassing other areas of the reservoir and reducing the overall sweep efficiency.

What is the total sweep of a reservoir, and how is it calculated?

The total sweep of a reservoir is the percentage of the volume filled with oil that has been contacted by the injected fluid in the connected segments. It is calculated by multiplying the vertical sweep by the aerial sweep.

How is secondary oil recovery using gas different from water injection, and what additional challenges does it face?

Gas injection in secondary oil recovery is different from water injection as gas can expand and create more pressure within the reservoir. However, it faces additional challenges due to the same effects of permeability variation, slumping, and preferential flow paths, which can limit its effectiveness.

Why does the injected water tend to take the shortest path to the production well once it reaches it?

Once the injected water reaches the production well, it tends to take the shortest path due to the established flow path, and subsequent water is more likely to follow this path rather than spreading out and sweeping more rock.

What factors can limit the effectiveness of water or gas injection in secondary oil recovery?

Factors that can limit the effectiveness of water or gas injection include non-uniform permeability, gravitational slumping, preferential flow paths, and the tendency for fluids to follow established paths rather than spreading out to sweep more of the reservoir.

How can understanding reservoir sweep behavior improve oil recovery strategies?

Understanding reservoir sweep behavior can help in optimizing injection strategies, such as adjusting injection points or rates, to ensure a more uniform and effective sweep, thereby improving overall oil recovery.